1. Field of the Invention
The present invention is directed generally to wireless communication devices and, more particularly, to a variable impedance load for a variable gain radio frequency amplifier used in the output stage of a wireless communication device.
2. Description of the Related Art
The use of wireless communication devices, in applications such as wireless telephones, is widespread. The output stage of a wireless communication device includes radio frequency (RF) amplifiers. This is true of most wireless communication devices regardless of the form of modulation, such as AM, FM, and the like. A typical output stage 10 for a wireless communication device is illustrated in FIG. 1 and comprises an RF driver amplifier 12, a transmit filter 14, and an RF power amplifier 16. An output port of the RF driver amplifier 12 is coupled to an input port of the transmit filter 14. Similarly, an output port of the transmit filter 14 is coupled to an input port of the RF power amplifier 16.
An input 18 to the RF driver amplifier comes from other circuitry not illustrated in FIG. 1. Those skilled in the art will appreciate that the other circuitry not illustrated includes, by way of example, processing circuits such as a modulator. For example, the wireless device may be a code division multiple access (CDMA) wireless communication device. In this example, the additional circuitry, which is not illustrated in FIG. 1, would include CDMA processing circuitry and a CDMA modulator circuit.
An output from the RF power amplifier 16 is coupled to a duplexer 20, the output of which is coupled to an antenna circuit, which is also omitted from FIG. 1 for the sake of brevity. Those skilled in the art will appreciate that the antenna circuit may comprise an antenna (not shown). The duplexer 20 permits the antenna to be used for both transmission and reception of radio frequency signals.
In an exemplary embodiment, the transmit filter 14 is a band pass filter selected to match the frequency range of operation of the wireless communication device. The transmit filter 14 may be implemented as a SAW filter or a ceramic filter. Those skilled in the art will understand the requirements of the transmit filter 14, which need not be described in greater detail herein.
For maximum efficiency in power transfer, it is desirable that the output impedance of the RF driver amplifier 12 match the input impedance of the transmit filter 14. Similarly, the output impedance of the transmit filter 14 should match the input impedance of the RF power amplifier 16 for maximum power transfer. If the RF driver amplifier 12 has a fixed gain associated therewith, it is within the knowledge of one skilled in the art to design an amplifier to have the appropriate output impedance to match the input impedance of the transmit filter. Alternatively, a matching circuit (not shown) may be inserted between the RF driver amplifier 12 and the transmit filter 14.
Failure to properly match the output impedance of the RF driver amplifier 12 to the input impedance of the transmit filter 14 results in a high voltage standing wave ratio (VSWR) and sub-optimal power transfer between the RF driver amplifier and the transmit filter. The impedance mismatch also adversely affects the operation of the transmit filter 14 resulting in a degradation of the in-band and out-of-band characteristics of the transmit filter. This mismatch may result in increased filter insertion loss, reduced filter gain flatness, and degraded out-of-band filter rejection.
Furthermore, the filter impedance at the output port of the transmit filter 14 (where the power amplifier 16 is coupled) is a function of the impedance seen by the transmit filter 14 at its input port. This input impedance must be carefully controlled in order to ensure the stability, linearity, and efficiency of the RF power amplifier 16. Thus, proper control of the output impedance of the RF driver amplifier 12 is critical to operation of the components following the driver amplifier in the transmitter chain (i.e., the transmit filter 14 and the RF power amplifier 16).
In known implementations of a wireless communication device, the gain of the RF driver amplifier 12 is not continuously varied in any significant way. In certain applications, the current drain of the RF driver amplifier 12 is altered to save current and thus maximize battery life. One side effect of such a decrease in the drain current is a minimal fluctuation on the gain of the RF driver amplifier, which is considered to be essentially a fixed gain. However, in applications where it is desirable to significantly vary the gain of the RF driver amplifier, there is a great need for a system and method to properly match the output impedance of the driver amplifier and thus improve performance of the output stage 10. The present invention provides this and other advantages as will be apparent from the following detailed description and accompanying figures.